a(n) = 3^(n-1) - 2^n + 1 (essentially Stirling numbers of second kind).

Terms

a(0) =0a(1) =0a(2) =2a(3) =12a(4) =50a(5) =180a(6) =602a(7) =1932a(8) =6050a(9) =18660a(10) =57002a(11) =173052a(12) =523250a(13) =1577940a(14) =4750202a(15) =14283372a(16) =42915650a(17) =128878020a(18) =386896202a(19) =1161212892a(20) =3484687250a(21) =10456158900a(22) =31372671002a(23) =94126401612a(24) =282395982050a(25) =847221500580